1. Field of the Invention
This invention relates to improved boron nitride evaporating crucibles. More particularly, this invention relates to improved pyrolytic boron nitride crucibles suitable for the vaporization of aluminum and other metals which wet boron nitride in their liquid state.
2. Description of the Prior Art
Pyrolytic boron nitride crucibles are used for the vacuum vaporization of metals and the deposition of thin films of such metals on various objects. Such procedure, of course, involves the use of the crucibles at highly elevated temperatures followed by subsequent cooling of the crucibles to room temperature. However, because of marked differences between the coefficients of thermal expansion of such crucibles and the metals vaporized therein, the crucibles often exhibit poor thermal cycling characteristics and a very high rate of catastrophic collapse upon cooldown subsequent to metal evaporation. Thus, for example, in the case of aluminum vaporization, about 80 percent of the crucibles implode at a temperature between 250.degree. C. and room temperature upon the first cool-down. This collapse is a result of the strong adhesion of the aluminum to the crucible walls and the contraction of the aluminum upon cooling at a rate about 20 times greater than that of the crucible.